Dealkalinization of sheets of glass with low alkalines content

ABSTRACT

This invention relates to a glass sheet having a low content of alkaline oxides and alkaline-earth oxides, in the composition of which the sum of the alkaline and alkaline-earth oxides is less than or equal to 15%, with a sodium oxide content less than or equal to 10%, the values being expressed in percentages by weight, at least one of the faces of which is constituted of a surface zone virtually devoid, notably, of alkaline ions.

This invention concerns glass products, notably sheets of glass with alow alkalines content, notably of the borosilicate type.

The composition of these glasses is such that the sum of the alkalineoxides and alkaline-earth oxides is less than or equal to 15%, with asodium oxide content less than or equal to 10%. These glasses havespecial applications, for example, in the field of reinforcing fibres orelectronics.

The characteristics, notably the thermal behaviour, of these glassesmake them interesting for serving as supports in the electronics sector.In particular, these glasses are used for the production of screens. Inthese applications it is necessary to protect the supported materialsagainst the possible contaminations that may result from the migrationof alkaline ions and alkaline-earth ions originating from the support.This is the case, for example, when the materials deposited on the glasssubstrate are of the semiconductor type and the migration of the mobileions appreciably alters their essential qualities.

For avoiding this type of difficulty, in the electronics field, it hasbeen proposed to use sheets of glassy the composition of which isvirtually devoid of alkaline and alkaline-earth oxides. Theseglass-making compositions may be such that the sum of the alkalineoxides and alkaline-earth oxides, mainly sodium oxide, is between 500and 1,000 ppm. These glass sheets server notably, as substrates for flatscreens. used in electronics.

It is known that the alkaline oxides have an important influence uponthe properties of the vitrifiable mixture; in particular, they enablethe viscosity at a given temperature to be reduced and the melting ofthe mixture to be improved. For these reasons a glass-making compositionhaving a low alkaline content presents many difficulties in processing.

Another solution to the problem of the migration in glasses containingalkaline ions and alkaline-earth ions is to deposit a surface film basedupon silica. This deposition may be carried out by pyrolysis, CVD,deposition under vacuum etc. This technique does, however, require anexcellent preparation of the surface of the glass sheet intended to becovered with this film, in order to remove any pollution. Thispreparation is time-consuming and meticulous. Furthermore, even anexcellent preparation does not enable the risks of delamination, whichwill be discussed later, to be averted.

An objective of the invention is to propose a glass which,simultaneously, offers a certain facility of processing and does notallow the alkaline ions and/or alkaline-earth ions forming part of itscomposition to migrate.

Another objective of the invention is to propose a glass sheet capableof replacing the glass sheets in which the composition is virtuallydevoid of alkaline ions, for particular applications such aselectronics.

Another objective of the invention is to propose the means of productionof this glass.

The present invention concerns a sheet of glass having a low content ofalkaline ions and alkaline-earth ions, at least one of the faces ofwhich has a surface zone virtually devoid of alkaline ions.

A glass sheet according to this invention is constituted of a glass, inthe composition by weight of which the sum of the alkaline andalkaline-earth oxides is less than or equal to 15% with a sodium oxidescontent less than or equal to 10%.

The alkaline oxides content is, preferably, according to this inventionless than or equal to 8%. Although the use of glasses having a lowcontent of alkalines is satisfactory according to this invention, thiscontent must not be too low so that the glass can be processed, as willbe explained later. It is necessary, in fact, for the glasses inquestion to have a sufficient ionic conduction at the processingtemperatures envisaged. To obtain a satisfactory conduction, the contentof alkaline ions is, preferably, not less than 0.1%. The content ofalkaline ions is, with advantage, between 2 and 8%.

In view of the low alkaline content and in order to maintain relativelylow fusion temperatures, it is preferable for the glass to have arelatively high content of boron oxide. This content is ordinarily notless than 5% and, preferably, not less than 6%. The boron oxide contentremains, generally, less than about 18%.

A preferred composition according to this invention is of the typecontaining the following principal constituents, the contents of whichare expressed in percentages by weight:

    ______________________________________                                                SiO.sub.2                                                                           80-85%                                                                  B.sub.2 O.sub.3                                                                     12-14%                                                                  Al.sub.2 O.sub.3                                                                    1-3%                                                                    Na.sub.2 O                                                                          3-5%                                                            ______________________________________                                    

Other preferred compositions according to this invention are of thefollowing type, the principal constituents being expressed inpercentages by weight:

    ______________________________________                                               SiO.sub.2                                                                              70-75%                                                               B.sub.2 O.sub.3                                                                         9-10%                                                               Al.sub.2 O.sub.3                                                                       4-6%                                                                 Na.sub.2 O                                                                             5-6%                                                                 K.sub.2 O                                                                              0-1%                                                                 BaO      3-4%                                                                 CaO      0-1%                                                                 ZnO      2-4%                                                                 or of the type:                                                               SiO.sub.2                                                                              68-75%                                                               B.sub.2 O.sub.3                                                                        12-13%                                                               Al.sub.2 O.sub.3                                                                       5-7%                                                                 Na.sub.2 O                                                                             6-7%                                                                 K.sub.2 O                                                                              1-2%                                                                 BaO      2-3%                                                                 CaO      0-1%                                                          ______________________________________                                    

These glass sheets generally have a coefficient of thermal expansionbetween 30 and 50×10⁻⁷ K⁻¹.

In certain applications, notably in the electronics field, it isadvantageous for the glass sheet and the supported films to havecoefficients of thermal expansion that are close together, in order toavoid any risk of delamination under the effect of temperaturevariations, such as those which are used, notably, at the time offorming these films. In the case of the production of a high-resolutionscreen (of the type polycrystalline silicon thin film transistor liquidcrystal display) the coefficient of thermal expansion of the supportedfilm is of the order of 40×10⁻⁷ K⁻¹, which is compatible with asubstrate having a coefficient of thermal expansion as indicated above.

The surface zone of the sheet according to this invention does notcontain more alkaline ions than the glasses known as "alkaline-free",which have been discussed above and this content may even besubstantially lower. Thus the alkaline ions content at the surface ofthe glass is, preferably, less than 500 ppm or can even reach a valueless than or equal to 50 ppm.

These contents are those in a surface zone which, ordinarily, does notexceed 1 micron in depth and, most commonly, is less than 0.5 micron.

In the intended uses, notably as electronic screens, a depth of thisorder of magnitude is very largely adequate for preventing the migrationof the ions into the semiconducting films or into the active filmssupported by the glass sheet. Provided that this layer is highlyuniform, depths less than 0.5 micron are also very satisfactory. Thesheets obtained according to this invention have a surface zone. withinwhich the sodium oxide content is less than 500 ppm, and indeed lessthan 50 ppm over at least 0.2 μm.

A significant test for quantifying the resistance of the substrate tothe migration of these ions is the test known as "rerelease" test. Thisconsists of measuring the sodium oxide content released by the substrateafter a dwell time of 24 hours in demineralized water at 96° C.

The glass sheet according to this invention releases a proportion of itssodium constituent less than 0.01 μg/cm².

For superficially deionizing glasses, it is possible to operate bychemical method. The glass article is subjected to a solution or anatmosphere that reacts with the alkaline ions of the glass. Tofacilitate the reaction, the operation is carried out at a temperaturehigher than ambient temperature. This type of treatment usually requiresa contact time that is incompatible with the continuous treatment ofglass sheets. For this reason, this type of treatment is used mainly inthe case of batches of articles, for example bottles or flasks.

Another technique is the deionization under the effect of an electricfield applied between two electrodes. The application of the fieldmobilizes the ions which are most easy to displace, in particular thealkalines, towards the cathode. Solid or gaseous electrodes may be used.

Direct contact between the electrode and a glass sheet does not favour auniform treatment. The use of electrodes at a short distance from theglass sheet and the formation of a "corona discharge", also known by theterm "plasma", on the contrary make possible a high uniformity andstability of the ion flux over the entirety of the sheet.

This technique has hitherto been envisaged for the treatment of glass ofthe silico-sodo-calcic type, that is to say glass containing a highcontent of alkaline and alkaline-earth oxides, exceeding 15% and,notably, a sodium oxide content greater titan 12%. The production of acorona discharge on a glass sheet implies a certain conductivity of thissheet in the operating conditions chosen. At ambient temperature,glasses are not good enough conductors to allow the establishment ofsuch a discharge. In the casa of the sodo-calcic glasses, the migrationof the most mobile ions and, notably, of the alkaline and alkaline-earthions, becomes sufficient at temperatures higher than approximately 450°C.

In these conditions, a substantial reduction of the content of theseions in the composition of the glasses treated inevitably ran counter tothe use of a deionization technique. In fact, the possibility ofcompensating the low content of mobile ions by an increased mobility ofthese ions appeared difficult to achieve. A raising of the treatmenttemperature came up against the question of possible softening of thesheet. The increase of the applied voltage raised other difficulties.For example, the increase in the voltage is limited by the formation ofelectric arcs, which do not allow a uniform treatment.

Contrary to what might have been expected, the inventors have shown thata glass sheet having a low content of alkaline ions and alkaline-earthions, as defined above, is capable of being deionized at the surface bythe corona discharge process.

In spite of their low proportion, the ions which are mobile under theeffect of a field of intensity comparable to that used previously withsodo-calcic glasses are capable of diffusing within the body of theglass.

For the production of the glass sheets according to this invention, theinventors have found the conditions that are most suitable for thedesired result in the use of the technique of deionization by plasma.

According to this invention, a d.c. voltage is preferably applied. Thisvoltage is a function of various factors. including the amount ofdeionization desired, and also the treatment temperature, the distancebetween the electrodes and the sheet, the speed of travel etc.

For obtaining a sufficient treatment in industrially practicable periodsof time, it is desirable to operate at a sufficiently high voltage. Thisvoltage is limited to that for which the risk of the development ofelectric arcs would be too great. In the opposite direction, it isnecessary, in order to initiate the formation of the discharge, to applya voltage which is not too low.

In the preferred conditions, notably, of temperature, distance from theelectrodes, speeds of travel and so on, which are examined below,advantageous mean voltage values between the electrodes lie, forexample, between 650 and 1,300 V and, preferably, between 900 and 1,100V.

In view of the voltage regulation carried out as indicated above, theintensity of the current discharge becomes established as a function ofthe electrical conduction of the assembly situated between theelectrodes. It is obvious that the temperature of the treated sheet isamong the most important factors. In view of the low content of mobileions and of the low conductivity property resulting therefrom, it ispreferably to operate at a temperature of at least 450° C. and,preferably, higher than 500° C. The temperature of the glass is not,ordinarily, higher than 650° C. and, preferably, is not higher than 600°C., in order to avoid a possible softening of the glass.

The inventors have shown that, depending upon the applied voltage andthe speed of treatment, the composition of the deionized surface variessubstantially. As an indication, a prolonged treatment and a largequantity of electricity passing through the specimen corresponding, forexample, to a succession of deionizations by repeated passage of theglass sheet through the electric field, leads to a relatively deepdeionized zone. In comparison, surprisingly, a treatment correspondingto a lesser quantity of electricity passing through the specimen in ashorter time, for example by an increase of the applied voltage and,simultaneously, a reduction in the number of passes. can lead to adeionization which is indeed less deep but for which the residualcontents of mobile ions on the treated surface are lower thanpreviously. This result is particularly favourable for a laterelectronic application, notably as substrates for flat screens. Thenumber of passes through the electric field, established in the voltageconditions indicated above, is then preferably between 1 and 5.

Experience has shown that compromises are necessary between the speed ofpassage and the applied voltage, depending upon the result desired. Itappears, in particular, that an increase above approximately 1,200 V inthe temperature conditions specified above does not allow a speedleading to an optimum treatment to be used. In these conditions, as weshall see from the examples of operation, the speed of passage whichwill avoid the risk of arc formation becomes too high for an intensetreatment. This speed is, preferably, between 1 and 3 m/min.

In the light of the operating conditions developed above, the quantityof electricity passing through the specimen during the treatment isadvantageously greater than 20 mC/cm² for obtaining a good deionizationaccording to the invention. It preferably lies between 20 and 30 mC/cm².

Details of the tests and advantages achieved according to this inventionare given in the remainder of this description, in which:

FIG. 1 is a graph showing measurements of the sodium oxide content inthe surface zone of the specimen according to the voltage applied andthe number of passages of the specimen through the electric field,

FIG. 2 is a graph of measurements of the sodium oxide content within thesurface zone of the specimen according to the applied voltage, thenumber of passes undergone by the specimen being fixed at 2,

FIG. 3 is a graph of measurements of the sodium oxide content in thesurface zone according to the applied voltage, the number of passesundergone by the specimen fixed at 1,

FIG. 4 is a graph of measurements of the calcium oxide content in thesurface zone before and after deionization by corona discharge accordingto this invention,

FIG. 5 represents the structure of the deionized surface zone of a glassaccording to this invention and of an alkaline-free glass.

A comparative example illustrates the advantages of the inventioncompared with a glass of the type known as "alkaline-free".

All these graphs have been obtained by the analysis of the mass spectrumof the ions sputtered by ion bombardment (technique known by theabbreviation SIMS).

Three series of tests were performed, the operating conditions being asfollows:

dimension of the glass sheet: 300×300 mm,

temperature of the sheet: 550° C.,

nature of the plasma-forming gas: helium or argon.

The electrodes are situated on either side of the glass sheet and at asmall distance from the sheet of the order of a few millimeters. Thecorona discharge is regulated in voltage.

The first series shows the influence of the treatment by coronadischarge and of the operating conditions on the proportion of sodiumconstituent present in the surface zone of a glass sheet tested,compared with a reference specimen no. 0. This specimen has undergone athermal cycle--30 min. at 550° C.--in order to place it in the sameconditions as those of the specimens treated by corona discharge.

Specimen no. 1 has undergone fifteen passes through the electric fieldunder a voltage across the electrodes of between 950 and 1,070 V;specimen no. 2 has undergone four passes through an electric field,under a voltage across the electrodes of from 650 to 740 V.

Repeated passes of the specimen through an electric field enable boththe treatment time of the specimen and also the current intensitypassing through the specimen to be increased.

As an indication, the quantity of charges measured relative to specimenno. 1 is 66 mC/cm² and is 25 mC/cm² for specimen no. 2.

In this series of tests, in addition to the voltage it is important topoint out the simultaneous modification in the speed of travel. This ismade necessary by the fact that, since the conduction of the glassvaries appreciably with the voltage applied, it is appropriate toincrease the speed of passage of the glass sheet undergoing test inorder to prevent the formation of electric arcs. This speed is 1 m/min.for a voltage across electrodes of 650 to 742 V and is 3 m/min. for avoltage across electrodes of 950 to 1,070 V.

The composition of the glass used is as follows, the proportions of thevarious elements being expressed in percentages by weight:

    ______________________________________                                                SiO.sub.2                                                                           81%                                                                     B.sub.2 O.sub.3                                                                     13%                                                                     Na.sub.2 O                                                                           4%                                                                     Al.sub.2 O.sub.3                                                                     2%                                                             ______________________________________                                    

FIG. 1 illustrates these results. The ordinate axis represents thecontent of the Na component to a logarithmic scale, expressed in ppm,and the abscissa axis represents the depth of the specimen tested,expressed in microns. Curves 0, 1 and 2 correspond respectively tospecimens 0, 1 and 2. The reference specimen no. 0 possesses arelatively high content of sodium oxide (40,000 ppm) by comparison withcurves 1 and 2, which shows the effectiveness of the corona dischargetreatment according to this invention on glasses having a low alkalinescontent.

Curves 1 and 2 exhibit different profiles.

Curve 2 shows a relatively abrupt and intense variation in the sodiumcomponent content within the surface zone, compared with curve 1.

The results are grouped in the attached table. These results aresatisfactory. The parameters investigated relating to the structure ofthe deionized zone are the depths of the deionized zone for a sodiumoxide content less than 500 ppm and less than 50 ppm. These contentshave been chosen in comparison with the glasses known as alkaline-free,in which the sodium oxide content is between 500 and 1,000 ppm. Thevalues obtained in the three series of tests are summarized in theattached table.

Surprisingly a zone is found (FIG. 1) in which the deionization of thesodium component is less thorough but relatively deep (2 μm) forprolonged treatment conditions (curve 1).

A more intense deionization is obtained (curve 2) for a treatment whichoverall is less thorough, taking account simultaneously of the differentparameters: voltage, speed and number of passes.

A compromise should therefore be found between these parameters.

In the light of these first tests, a second series of tests was carriedout at higher applied voltages between the electrodes of the order of1,075, 1,275 V, each specimen tested undergoing only two passes throughthe electric field. The speeds of travel are, respectively, 1.75 m/min.and 3 m/min. The other operating conditions are identical to thosedescribed above.

For a voltage of 1,075 V, the quantity of charges measured is 26 mC/cm²; for a voltage of 1,275 V, it is 21 mC/cm².

Surprisingly, a smaller quantity of charges passing through the specimenis obtained when the voltage is higher.

FIG. 2 illustrates the results obtained. Curve 0 represents a referencespecimen which has undergone the same thermal cycle as that described inthe first series of tests, for identical reasons. Curves 3 and 4(corresponding respectively to an applied voltage between the electrodesof 1,075 and 1,275 V) exhibit different profiles, and in particularcurve 3 demonstrates a higher deionization than that of curve 4. Thesodium oxide content is higher within the deionized layer for a highervoltage (1,275 V) compared with a less high voltage (1,075 V).

Furthermore, the depth of the deionized layer for the higher voltage(1,275 V) is not significant.

The form of this curve can be explained, according to the inventors, bythe increase in the speed of passage simultaneously with the increase inthe voltage. This increase was found necessary in order to prevent theformation of any electric arcs; it therefore appears too high for anintense treatment.

The results are summarized in the attached table. These results aresatisfactory, a sodium component content less than 500 ppm, and indeedless than 50 ppm for specimen no. 3, being obtained.

FIG. 3, illustrating a third series of tests, confirms the conclusionsreached by observation of FIGS. 1 and 2. Curves 5 and 6 relate tospecimens 5 and 6, corresponding to applied voltages across theelectrodes of 990 and 1,040 V respectively.

This series of tests was performed in the same conditions as before,except for the number of passages of each specimen through the electricfield (in this case it is one). The quantities of charges measured are,respectively, 21 mC/cm² and 16 mC/cm².

The results are practically identical to those obtained in the secondseries of tests. A slight decrease will be noted in the depth of thedeionized layer with the change from two passes to one pass, which is inagreement with the conclusions previously reached, that is to say thatan increase in the number of passes of the specimen through the electricfield increases overall the thickness of the deionized zone.

In the light of these experiments, a very clear influence of theoperating conditions upon the structure of the deionized zone can beseen. These conditions can be adapted according to the envisagedapplication, while still allowing for the physical constraints resultingfrom the operating mode adopted such as, for example, the number ofpasses to be performed for each specimen and the speed of passage of theglass sheet. These two parameters particularly have an effect upon thetreatment time of each specimen.

For an application in electronics as an electronic support, a goodcompromise is a voltage across the electrodes of the order of 1,000 V, aspeed of passage of approximately 1.75 m/min. and two passes of aspecimen through the electric field. The quantity of charges thenpassing during the treatment is of the order of 25 mC/cm².

FIG. 4 shows the deionization obtained according to this invention, notonly of alkaline oxides but also of alkaline-earth oxides.

Curves 7 and 8 correspond, respectively, to the calcium oxide content ofa reference specimen which has undergone a thermal cycle--30 min. at550° C.--and of a specimen obtained in the same operating conditions asthat of curve 3 of FIG. 2 (average voltage across the electrodes 1,075V; speed of travel approximately 1.75 m/min.; two passes, glasstemperature 580° C.).

Curve 3, corresponding to the sodium oxide content of this samespecimen, is repeated for purposes of comparison.

The results are summarizedbelow:______________________________________Depth of deionized zonecorresponding to a content of Curve 7 Curve8______________________________________Calcium oxide less than 500 ppm(in μm) >1 0.62Calcium oxide less than 50 ppm (in μm) --0.40______________________________________

An effective deionization with respect to calcium oxide is observedaccording to the process of this invention.

By comparing curves 3 and 8 relating to sodium oxide and calcium oxiderespectively, it will be seen that calcium oxide, although less mobilethan sodium oxide,has diffused equally well, and indeed better, withinthe body of the glass under the effect of the corona discharge.

EXAMPLE 1

The following example illustrates the advantages of this invention withrespect to a glass known as "alkaline-free".

Three glass sheets are subjected to the so-called "rerelease" test forsodium. This test consists of immersing each of the glass sheets in ademineralized water at 96° C., for which the content of the sodiumcomponent is known. After 24 hours, the water is collected and thesodium content is measured. From the difference, the content of sodiumreleased by each of the glass sheets is obtained.

A sheet A is formed of a known glass, termed "alkaline-free", themeasured chemical composition of which is as follows, the contents beingexpressed in percentages by weight:

    ______________________________________                                               SiO.sub.2                                                                            46.7%                                                                  B.sub.2 O.sub.3                                                                      14.8%                                                                  Al.sub.2 O.sub.3                                                                     12.7%                                                                  Na.sub.2 O                                                                            0.1%                                                                  BaO    24.9%                                                                  As.sub.2 O.sub.3                                                                      0.3%                                                                  Impurities                                                                            0.5%                                                           ______________________________________                                    

The other two sheets are produced according to this invention under theoperating conditions below.

The speed of travel is 1.75 m/min.

The values of the average voltage between the electrodes and the numberof passages of sheet B are respectively 1,050 V (between 900 and 1,070)and 19 passes.

The values of the mean voltage between the electrodes and the number ofpasses for sheet 3, in accordance with curve 3 of FIG. 2, arerespectively 1,075 V (voltage between 1,030 and 1,120 V) and 2 passes.

The upper operating conditions are identical to those described inrelation to the Figures.

The sodium component contents measured are as follows:

    ______________________________________                                        Sheet A (glass known as alkaline-free):                                                             0.015    μg/cm.sup.2                                 Sheet B (according to the invention):                                                               0.01     μg/cm.sup.2                                 Sheet 3 (according to the invention):                                                               0.008    μg/cm.sup.2                                 ______________________________________                                    

According to the invention, the deionized layer constitutes a betterbarrier against the migration of the sodium constituent than thatobtained with a glass known as alkaline-free.

FIG. 5 illustrates the structure of the deionized zone of sheet 3 and ofthe alkaline-free glass.

Curves 3 and 8 correspond to the sodium oxide and calcium oxide contentsrespectively of sheet 3.

Curves 9 and 10 correspond to the sodium oxide and calcium oxidecontents respectively of the glass known as alkaline-free.

The results are summarized below:

    ______________________________________                                        Depth (in μm) of    Glass known as                                                                           Sheet                                       the deionized zone corresponding to a content of                                                     alkaline-free                                                                            3                                           ______________________________________                                        Calcium oxide less than 500 ppm                                                                      >1         0.62                                        Calcium oxide less than 50 ppm                                                                       --         0.40                                        Sodium oxide less than 500 ppm                                                                       0.007      0.56                                        Sodium oxide less than 50 ppm                                                                        --         0.36                                        ______________________________________                                    

The surface zones of the sheets produced according to this inventionexhibit contents of sodium oxide and calcium oxide less than thosepresent in the glass termed alkaline-free, commonly used in theelectronics field. The glass sheets according to this invention canserve as substrates for the production of flat screens in theelectronics field, notably flat screens of the type LCD (liquid crystaldisplay) or of the electroluminescent type, or even flat screens havingan active matrix termed "active matrix liquid crystal display".

EXAMPLE 2

This example illustrates the difficulty of deionizing a glass having alow alkalines content.

A glass sheet, made of a glass termed alkaline-free in which thecomposition is identical to the composition described in Example 1, issubjected to a corona discharge according to this invention.

In order to obtain an intense treatment, the operating conditions are asfollows:

    ______________________________________                                        applied voltage between electrodes:                                                                2,600    V                                               charges quantity measured:                                                                         0.5      mC/cm.sup.2                                     speed of travel:     2        m/min.                                          number of passes:    36                                                       ______________________________________                                    

A surface zone is obtained comprising a sodium oxide content of 300 ppmto a depth of 0.5 μm.

In spite of the particularly severe operating conditions, a deionizationis obtained which is relatively poor compared with the values obtainedaccording to this invention, which can reach a content less than 50 ppmto at least 0.2 μm.

This example illustrates the difficulty of deionizing glasses that arevirtually devoid of alkalines, the quantity of charges passing throughbeing almost non-existent: the initial alkaline content in the glass isan important parameter for carrying out the process according to theinvention.

                  TABLE                                                           ______________________________________                                                       Specimen                                                                        1        2        3                                          ______________________________________                                        Operating conditions                                                          Voltage applied between elec-                                                                  950-1,070                                                                              650-740  1,030-1,120                                trodes (V)                                                                    Mean voltage applied between the                                                               1,050    695      1,075                                      electrodes (V)                                                                Number of passages through the                                                                 15       4         2                                         electric field                                                                Speed of travel (m/min.)                                                                        3       1        1.75                                       Quantity of charges measured                                                                   66       25       26                                         (mC/cm.sup.2)                                                                 Structure of deionized zone                                                   Depth of deionized zone in which                                                               2.6      1.0      0.56                                       the sodium oxide content is less                                              than 500 ppm (μm)                                                          Depth of deionized zone in which                                                                0       0.6      0.36                                       the sodium oxide content is less                                              than 50 ppm (μm)                                                           ______________________________________                                                       Specimen                                                                        4        5        6                                          ______________________________________                                        Operating conditions                                                          Voltage applied between elec-                                                                  1,230-1,320                                                                            990      1,040                                      trodes (V)                                                                    Mean voltage applied between the                                                               1,275    990      1,040                                      electrodes (V)                                                                Number of passages through the                                                                 2         1          1                                       electric field                                                                Speed of travel (m/min.)                                                                       3        1.75        3                                       Quantity of charges measured                                                                   21        21        16                                       (mC/cm.sup.2)                                                                 Structure of deionized zone                                                   Depth of deionized zone in which                                                               0.45     0.43     0.30                                       the sodium oxide content is less                                              than 500 ppm (μm)                                                          Depth of deionized zone in which                                                               0.06     0.22     --                                         the sodium oxide content is less                                              than 50 ppm (μm)                                                           ______________________________________                                    

We claim:
 1. A low alkaline oxide and alkaline-earth oxide contentborosilicate glass sheet comprising:at least one face comprising asurface zone within which the alkaline ion content is less than or equalto 500 ppm to a depth of at least 0.2 μm; and a compositioncomprising:sodium oxide; alkaline oxides other than sodium oxide;alkaline-earth oxides; and boron oxide; wherein the sum of said sodiumoxide, said alkaline oxides other than sodium oxide, and saidalkaline-earth oxides is less than or equal to 15% by weight of thetotal glass sheet; wherein the amount of said sodium oxide is less thanor equal to 10% by weight of the total glass sheet; wherein said boronoxide content is not less than 5% by weight of the total glass sheet;and wherein said glass sheet has a coefficient of thermal expansion ofbetween 30-50×10⁷ K⁻¹.
 2. The glass sheet of claim 1, wherein saidalkaline oxide content of said composition is 2 to 6% by weight of thetotal glass sheet.
 3. The glass sheet of claim 1, wherein said boronoxide content is 5 to 18% by weight of the total glass sheet.
 4. Theglass sheet of claim 1, wherein said boron oxide content is 6 to 15% byweight of the total glass sheet.
 5. The glass sheet of claim 1, whereinsaid composition comprises:

    ______________________________________                                                SiO.sub.2                                                                           80-85%                                                                  B.sub.2 O.sub.3                                                                     12-14%                                                                  Al.sub.2 O.sub.3                                                                    1-3%                                                                    Na.sub.2 O                                                                          3-5%                                                            ______________________________________                                    

wherein the percentages are expressed as percentages by weight of thetotal glass sheet.
 6. The glass sheet of claim 1, wherein saidcomposition comprises:

    ______________________________________                                                SiO.sub.2                                                                           70-75%                                                                  B.sub.2 O.sub.3                                                                      9-10%                                                                  Al.sub.2 O.sub.3                                                                    4-6%                                                                    Na.sub.2 O                                                                          5-6%                                                                    K.sub.2 O                                                                           0-1%                                                                    BaO   3-4%                                                                    CaO   0-1%                                                                    ZnO   2-4%                                                            ______________________________________                                    

wherein the percentages are expressed as percentages by weight of thetotal glass sheet.
 7. The glass sheet of claim 1, wherein saidcomposition comprises:

    ______________________________________                                                SiO.sub.2                                                                           68-75%                                                                  B.sub.2 O.sub.3                                                                     12-13%                                                                  Al.sub.2 O.sub.3                                                                    5-7%                                                                    Na.sub.2 O                                                                          6-7%                                                                    K.sub.2 O                                                                           1-2%                                                                    BaO   2-3%                                                                    CaO   0-1%                                                            ______________________________________                                    

wherein the percentages are expressed as percentages by weight of thetotal glass sheet.
 8. The glass sheet of claim 1, wherein said alkalineion content is less than 50 ppm to a depth of at least 0.2 μm.
 9. Theglass sheet of claim 1, wherein the amount of said sodium oxide releasedfrom said glass sheet after a dwell time of 24 hours in a demineralizedwater at 96° C. is less than or equal to 0.01 μg/cm².
 10. The glasssheet of claim 9, wherein said glass sheet acts as a substrate uponwhich are deposited conducting films for the production of flat screensof the "liquid crystal display" type or screens of theelectroluminescence type.
 11. The glass sheet of claim 9, wherein saidglass sheet acts as a substrate upon which is deposited a fine networkof diodes or transistors or metal/insulator/metal structure forming flatscreens of the "active matrix liquid crystal display" type.
 12. Aprocess for deionizing the surface of a low alkaline oxide andalkaline-earth oxide content borosilicate glass sheetcomprising:applying a corona discharge created by the effect of anelectric field applied between two electrodes to said glass sheet with acoefficient of thermal expansion of between 30 and 50×10⁷ K⁻¹ whereinthe sum of the alkaline oxides and alkaline-earth oxides is less than orequal to 15% by weight of the total glass sheet; the sodium oxidecontent is less than or equal to 10% by weight of the total glass sheet;the boron oxide content is not less than 5% by weight of the total glasssheet; and said surface has an alkaline ion content less than or equalto 500 ppm to a depth of at least 0.2 μm.
 13. The process of claim 12,wherein said corona discharge is created by the effect of a currentapplied by means of a voltage-regulated generator between twoelectrodes.
 14. A process according to claim 13, wherein saidvoltage-regulated generator produces a mean voltage between theelectrodes of between 650 and 1,300 V.
 15. A process according to claim14, wherein said process generates a quantity of electricity greaterthan 20 mC/cm² and preferably between 20 and 30 mC/cm².